The present invention relates to a digital signal reproducing apparatus suitable for reproducing digital PCM audio signals that have been recorded in the form of single helical tracks on a recording medium, one track being formed per unit time, with a rotary head.
A technique is known in which audio signals are recorded on magnetic tape in the form of helical tracks by helical scanning with a rotary head, one track being formed per unit time, and being reproduced subsequently. A digital signal recording/reproducing apparatus known as an R-DAT (rotary head type digital audio tape recorder) has been designated for recording audio signals as PCM signals and subsequently reproducing the same.
One recorded track format in an actual R-DAT system is shown in FIG. 14A, in which each of MARGIN, PLL and POSTAMBLE has a frequency of 1/2 f.sub.m (f.sub.m =9.4 MHz) and IBG has a frequency of 1/6 f.sub.m. Each of SUB and PCM is composed of a plurality of blocks, as shown in FIG. 14B. SYNC is composed of 10 bits, of which 9 bits are fixed, with the remainder assuming various patterns depending upon the pace and audio signals. SUB consists of a cyclic pattern of 8 such blocks, and PCM has 128 blocks. The numerals given in FIG. 14A represent the numbers of blocks occupied by the respective regions. The regions ATF-1 (between SUB-1 and PCM) and ATF-2 (between PCM and SUB-2) (ATF=automatic track finding) are provided to ensure that tracking control, i.e., control for allowing a rotary head to correctly scan the recorded tracks during reproduction, can be accomplished by means of the output of the head itself without employing any special head.
In R-DAT, time base compressed PCM signals are recorded in the form of helical tracks on magnetic tape by means of two rotary heads. Instead of providing a guard band between adjacent tracks, a tracking pilot signal is recorded both at the beginning and at the end of each track in a region independent of the area in which the PCM signals are recorded. During reproduction, the recorded tracks are scanned with a rotary head having a scanning width larger than the width of each track, and the output of the reproduced pilot signals from the two tracks adjacent to the track being scanned is used to control the tracking of the rotary head.
The ATF track pattern is shown in FIG. 15 and is hereinafter described with reference to the case of a drum having a diameter of 30 mm which is rotating at 2,000 rpm with the tape wound at an angle of 90.degree. with respect to the drum.
ATF-1 and ATF-2, located respectively in the front and rear portions of each track, have a small azimuth effect signal f.sub.1 having a low frequency of 130 kHz (=f.sub.m /72) as a tracking pilot signal. This signal is used to detect the levels of crosstalk resulting from the two tracks adjacent to the track being reproduced, so as to obtain the difference between the levels of such crosstalk as a tracking error signal.
In each of ATF-1 and ATF-2 there is recorded a sync signal for identifying the location at which the pilot signal f.sub.1 is recorded. In the presence of crosstalk, the sync signal is unable to distinguish the current track from adjacent tracks, so it is selected in such a way that it not only has a frequency capable of producing an azimuth-effect but also affords a pattern different from that of the PCM signal. If the head having a + (plus) azimuth is designated A and the head having a - (minus) azimuth is designated as B, two different sync signals are provided to distinguish head A from head B. Stated more specifically, a sync 1 signal f.sub.2 having a frequency of f.sub.m /18 (=522 kHz) and a sync 2 signal f.sub.3 having a frequency of f.sub.m /12 (=784 kHz), respectively corresponding to head A and B, are recorded in predetermined positions.
In an R-DAT which does not employ an erase head, a new signal is written over the previously recorded signal. In order to enable this "overwrite" mode, an erase signal f.sub.4 having a frequency of f.sub.m /6 (=1.56MHz) is recorded at a predetermined position to erase the previously recorded pilot signal f.sub.1, sync 1 signal f.sub.2, and sync 2 signal f.sub.3.
The ATF pilot signals are located at different positions on the current track and the two adjacent tracks. The level of the pilot signal on the current track (i.e., the track being scanned) differs on a time basis from the level of each of the pilot signals on the adjacent tracks, so that the three different levels can be sampled independently of each other.
Five blocks are assigned to each of the ATF regions, ATF-1 and ATF-2, and the pilot signal f.sub.1 is recorded in two of these blocks. The sync signal f.sub.2 is recorded in an area covering 1 or 0.5 block from the center of the position in which one of the two other adjacent tracks is recorded. The pilot signal f.sub.1 on the other adjacent track is recorded in such a way that its center is positioned two blocks after the beginning of the sync signal recorded on the current track. A one-block sync signal is assigned to an odd-number frame, and a half-block sync signal is assigned to an even-number frame.
As described above, the sync signals to be recorded in the ATF region have different frequencies depending upon which head is used in scanning, and these sync signals also have different recording lengths in odd-number frames and even-number frames. This design enables four consecutive tracks to be distinguished from one another since they are provided with different ATF regions. Thus, the pattern of ATF regions is of the 4-track completed type, being cyclically repeated every 4 tracks.
When magnetic tape in which signals have been recorded in the format shown in FIG. 14A is played back with a rotary head, an RF signal of the type shown in FIG. 16A is reproduced from the head. If this RF signal is obtained by playback of a track with the odd-number frame (A) shown in FIG. 15, it may be passed through a bandpass filter (BPF) of 130 KHz so as to obtain a pilot signal f.sub.1 as shown in FIG. 16B.
The signal in zone I is due to the pilot signal on the current track, and those in zones II and III result from crosstalk of the pilot signal on a track with the odd-number frame (B) and a track with the even-number frame (B), respectively. If the rotary head were scanning the current track correctly, the envelope levels of zones II and II, or the values of V.sub.II and V.sub.III indicated in FIG. 16C should be equal to each other. However, if a tracking deviation occurs, V.sub.II is not equal to V.sub.III (V.sub.II .noteq.V.sub.III), and the amount and direction of the deviation of the rotary head with respect to the current track can be determined by the magnitude and polarity of the difference between V.sub.II and V.sub.III. Therefore, by actuating a capstan servo according to the difference between V.sub.II and V.sub.III so as to effect fine adjustment of the tape speed, the rotary head can be controlled to travel correctly on the current track.
However, as described before, R-DAT does not employ an erase head, and subsequent recording is carried out by overwriting. Therefore, it is sometimes not possible to generate a correct error signal upon correct detection of the sync signal and sampling of the value V.sub.II and V.sub.III.
Specifically, in R-DAT, recording may be performed with the range of .+-. two blocks from the center of the PCM region. Further, pilot signal f.sub.1 (=130 kHz) is recorded at a level slightly lower than the recording levels of the reminder signals. This is done in order to have the previously recorded pilot signal erase by the erasing signal, since the signal having a lower frequency is recorded on the tape with a deeper recording level. However, with the pilot signal f.sub.1 having a lower recording level, the previously recorded sync signal tends to remain unerased when the pilot signal f.sub.1 is newly recorded in place of the sync signals f.sub.2 and f.sub.3 which have previously been recorded.
More specifically, when new recording is carried out with a displacement in the forward direction with respect to the previous recording, there is no problem, since the sync signal of the new recording always precedes the sync signal of the previous recording which has remained unerased. However, a problem will be caused in the case in which the sync signals of the new recording are displaced in the backward direction and the unerased sync signal precedes the new sync signal. An example of such a problem is that the displacement occurs in the backward direction by an amount in the range of one to two blocks. Partial or entire sync signals f.sub.2 and f.sub.3 which previously have been recorded remain unerased in the portion of the pilot signal f.sub.1 in even-number frame (A) and odd-number frame (A) with respect to ATF-1 and in even-number frame (B) and odd-number frame (B) with respect to ATF-2.
If such a problem occurred, sampling would be implemented to sample a level of a frequency component of the pilot signal contained in the reproduced RF signal in response to the previously recorded sync signal. This pilot signal should have been at a crosstalk level of the sampling signal in one adjacent track. However, the sampled frequency component actually is that of the pilot signal of the current track, and the level obtained by sampling is extremely large. Thereafter, the frequency component of the pilot signal contained in the reproduced RF signal coming after two blocks is sampled, the difference in level of this sampled value and the sampled value obtained two blocks before is computed, and the capstan servo is controlled in accordance with this level difference as the amount of track deviation. However, since the previously sampled value is not the crosstalk level of the adjacent track but the level of the current track, an extremely large level difference compared with the actual track deviation is obtained. When such a phenomenon occurs, the capstan servo is disturbed and the tape travel is badly affected.
Although description has been provided for the case in which the previous sync signal remains unerased in the portion of the pilot signal which has been newly recorded, the sync signal may otherwise remain as noise as a result of incomplete erasure of the sync signal by the erase signal.